dSLRs may be a bit 'old hat' to the buying public (keep in mind that still about 50/50 split last year between mirrorless vs. dSLR new bodies), but dSLRs are typically significantly less expensive to buy and outfit...LOTS of used bodies and used lenses, and there are places that have significant inventories of rental lenses...a major consideration when you are going on safari but otherwise have little use for supertelephoto FL. It seems RF lenses are more expensive to buy than 'same' EF lenses, and there are fewer used RF on the market, too.
dSLR bodies like nD have shutter life of 150k or more, so if you find a used body with half that shutter count, it has considerable useful life left.
Consider buying an 'everyday usage' lens for the body, and renting the superlong tele. Or buy a used superlong tele and reselling it after you are done with it; easier to find used EF mount lenses that to find used RF mount lenses. You can always use a mount adapter to use EF tele lens on RF body.
Lastly, until there are more R7s around, you need to select dSLR for the 'reach' attainable with same FL lens, when mounted on a crop format body. While some mirrorless have a crop mode when crop format (e.g EFS) lenses are mounted, there is a significant loss of total pixel count when the body is in crop mode...a native APS-C format body has more pixel density than FF in crop mode.